1. Field of the Invention
This invention relates generally to a non-metallic device and method for the stabilization of at least a portion of a vertebral column and more particularly to a device and method for spinal stabilization and also which incorporates a microporous material.
2. Description of the Prior Art
Fusion of vertebral columns by instrumentation devices and/or bone material is common and a long practiced surgical technique. Fusion is the permanent internal fixation of part or all of the intervertebral joints, an intervertebral joint being composed of two adjacent vertebrae and their posterior bony elements connected by an intervertebral disc, ligaments, and two facet joint capsules. However, the concept of fusion carries significant and previously unappreciated patient disability. By fusing vertebrae the remaining segments are subject to inordinately high stress and degeneration. In the typical L4-S1 fusion the L3-4 level typically undergoes dramatic degeneration of the disc and zygoapophyseal joints leading to the production of a clinical entity called the "transitional syndrome" often leading to "mechanical-type" pain or actual spinal nerve compression. The stress patterns produced by fusion often produce clinical problems relating to the sacro-iliac and hip joints.
When spine fusions involve mechanical instrumentation, significant forces are directly aimed at the supportive sites whether they be bone screws, hooks, etc. This phenomenon usually produces loosening of the points of attachment for the implanted hardware and a resulting loss of support by this instrumentation. Because of this, fusions involving instrumentation are often carried out in conjunction with bone fusions so as the instrumentation loosens and fails, support can be maintained by growth of the bony counterpart. These combined procedures involve extensive surgery, substantial blood loss and high cost. Following such a procedure, patients are usually disabled for long periods of time.
Therefore, the treating of spinal instability by fusion is not optimal. It immobilizes adjacent segments, eliminating their previous natural ability to move relative to each other. Further, when fusion has taken place with the addition of internal fixation, often the internal fixation parts are of a metallic nature. In modern medical practice the presence of dense metallic devices is a significant liability due to the scattering artifacts generated if imaging is attempted. Since the spinal diagnostic procedure of choice is now imaging, the presence of significant metallic objects represents a serious diagnostic limitation.
In 1973 scientists at the Materials Research Laboratory at Penn State University observed that is some reef building corals their microstructure was characterized by 1) a high degree of uniformity in pore diameter, 2) a pore interconnection diameter similar in size to that of the pore itself, 3) a solid-to-volume ratio or approximately one, and 4) exceptionally high permeability in which each and every pore was interconnected to all other pores. It was recognized that it would be virtually impossible to achieve these structural characteristics by artificial means. Early experiments designed to impregnate the coral with various substances, dissolve the calcium carbonate coral skeleton and then make positive casts with other materials including metal alloys were successful and the term "the replamineform process" was coined.
In addition to replamineform casting it became evident that the structural characteristics of coral itself could possibly serve as the means for bone ingrowth after cleansing of organic materials and chemical conversion from biogenic carbonate to hydroxyapetite.
Subsequent medical studies using coral with a microdiameter (200-250 microns) similar to that of the normal human bone Haversian canals (200-400 microns) were conducted by a number of physicians in the United States. Drs. Vert Mooney and David Selby in Dallas, Tex. performed 40 cases of bilateral spinal bone fusion in which one side was coral provided by Interpore International of Irvine, Calif., and the other side autogenous bone. These studies did not demonstrate significant ingrowth of osteoblasts into the coral and these observations led to the conclusion that coral showed little value in this regard. Since this work most clinicians have almost exclusively used autogenous or donor derived homologous bone.
The present invention addresses the problems associated with the prior art devices and method for treating spinal instability and provides for stabilization, rather than fusion, that is, support with the allowance of some natural movement or flex, and accomplishes this with a minimal amount of metallic components and may allow for ingrowth by fibroblasts to occur.